Transmitting Return-to-Zero (RZ) pulses is currently in common use in terrestrial optical-fiber transmission systems. One of the problems encountered, in particular for existing systems, is that of increasing the data rate or the transmission distance without giving rise to error. Various solutions have been proposed. A first approach consists in reducing the duration of the RZ pulses, and in using time-division multiplexing. That approach is limited by the jitter caused by the propagation and by the various optical components of the transmission system. Similar problems are encountered for transmission systems using Non-Return-to-Zero (NRZ) pulses.
In order to mitigate that problem, it is known that the optical signal can be converted into an electronic signal in the regenerators and the electronic signal can then be regenerated. That approach is intrinsically limited by the pass-band of the semiconductor components used. It is also limited in terms of the maximum length of the transmission system. In addition, it is costly when the data rate is increased.
It has also been proposed to perform wavelength conversion, relative to a local clock of different wavelength and without jitter, by applying the RZ signal with jitter as a control signal to the clock. In addition to requiring a local clock, that technique also suffers from the drawback of being difficult to apply when wavelength-division multiplexing is used, or, more generally, when it is awkward to change wavelength.
Transmitting soliton pulses or solitons is known. Such pulses are RZ pulses of time width (Full Width at Half Maximum (FWHM)) that is narrow relative to bit time, they have a determined relationship between power, spectrum width, and time width, and they propagate generally in the "abnormal" dispersion portion of an optical fiber. The variation in the envelope of such a soliton pulse in a monomode fiber can be modelled by the non-linear Schrodinger equation: propagation relies on a balance between the abnormal dispersion of the fiber and its non-linearity. In order to control the jitter of such soliton signals, various solutions have been proposed. It is known that sliding guiding filter systems can be used (see, for example, EP-A-0 576 208). It has also been proposed to perform synchronous modulation on the soliton signals. For that purpose, it is possible to use modulators of various types, and in particular synchronous amplitude or phase modulators using the Kerr effect. Descriptions of the various techniques for controlling or regenerating soliton signals can be found in the article entitled "Soliton Transmission Control in Time and Frequency Domains" by H. Kubota and M. Nakazawa, IEEE Journal of Quantum Electronics, vol. 29, No. 3,2189, or in the article entitled "Evaluating the Capacity of Phase Modulator-Controlled Long-Haul Soliton Transmission" by N. J. Smith and N. J. Doran, Optical Fibers Technology I, 218-235 (1995).
Those techniques are not limited by the pass-band of electronic components. Unfortunately, they cannot be applied directly to non-soliton RZ pulses because the pulses or their spectrums are different from soliton signals.